A progressive cavity pump, also commonly known as a Moineau pump, is comprised of two interfacing helical components, namely, a stator and a rotor. Typically the stator comprises a cylindrical metal housing attachable to a tubing string and an elastomeric helical and longitudinally extending cavity mounted to the inside of the metal housing. Typically the rotor comprises a metal helical rod attachable to a rod string. As a general principle, the rotor has a helix having one helical order less than the stator i.e. the rotor has a helical order n and the stator has a helical order of n+1. For example, when the rotor is a single helix of helical order n=1, the stator has a double helix of helical order n=2, and when the rotor is a double helix with n=2, the stator is a triple helix with n=3, and so on. In such configurations open cavities exist within the pump. Rotating the rotor within the stator will cause these cavities to progress and to operate as a pump. Rotational means is typically provided by a motor, which drives the rotor via a rod string. The capacity for a progressive cavity pump to operate against a discharge pressure greater than the intake pressure is proportional to the number of stages within the pump. A stage is equal to one pitch length of the stator, and is defined by one revolution of the stator helix. For a given helix geometry, the pressure capacity of the pump increases as stages are added and the length of the pump increases proportionally. However, as the number of stages in a pump is increased, the required torque to drive the rotor is also increased since the pump becomes longer.
Progressive cavity pumps are particularly useful due to their capable handling of viscous and solid particulate laden fluids and have been deployed in a number of applications including transporting food, slurry, sewage and emulsions. An emulsion may consist of a number of different fluids including, but not limited to, a mixture of oil, water, sand and hydrocarbon gas. When pumping commonly ‘harsh’ fluids, the pump tends to wear over time to a point where it is no longer effective. Once a progressive cavity pump is no longer effective it must be replaced. In some applications, the cost to replace a progressive cavity pump can be prohibitive due to the cost of the pump parts as well as to the efforts undertaken to access the pump, and particularly the stator.
One application where accessing the stator is particularly challenging and costly is pumping in an oil or water wellbore. In wellbore applications, the pump is generally installed up to several thousand feet below ground level. Current practices for installing such a pump involve attaching the stator to the wellbore's tubing string and providing an inwardly protruding restriction in the tubing string either above or below the stator that is used to locate the rotor relative to the stator (known respectively as a “top locating” or a “bottom locating”); the tubing with the restriction and stator is then inserted into the borehole using a service rig. The rotor is attached to a rod string, which is inserted into the tubing string using the service rig; the rod string and rotor are lowered until contact is made with the restriction, at which point the rotor location relative to the stator is known and a rotor space out procedure may be completed. A variety of other tools can be attached to the rod string or tubing string without interfering with the inwardly protruding restriction or pump components.
Generally, progressive cavity pumps used in wellbores are manufactured and sold in lengths that provide the required pressure capacity, or lift, to bring fluid to surface. If a well operator is satisfied with the pressure capacity and geometry of a particular pump, he would typically only be concerned about the length of the pump if it approached or exceeded the limits required for installation or if torque was a potential problem. In general, the rod string and rotor can be retrieved and reinstalled by a smaller, less expensive unit than a service rig known as a flush-by unit. However, the flush-by unit is generally not capable of retrieving or installing the tubing string and stator and thus the service rig is again required when the pump has worn out and is in need of servicing/repair/replacement. The service rig is deployed to pull out the rod string and rotor, and then pull out the tubing string and stator. The worn stator is then replaced with a new stator and the service rig inserts the tubing string with new stator back into the wellbore.
The worn rotor is also replaced and the service rig inserts the rod string with new rotor back into the tubing string. Such work tends to take several hours at significant expense and lost production to the operator.